Open File Report OF-AR-18
New Mexico Bureau of Geology and Mineral Resources
A division of
New Mexico Institute of Mining and Technology
40
39
Prepared By:
New Mexico Bureau of Geology, Socorro, NM 87801
Prepared For:
Dept. of Earth and Space Sciences, University of California, Los Angeles, CA 90095
Initially prepared as:
NM Geochronology Research
Laboratory Internal Report
NMGRL-IR 186
February 4, 2002
SOCORRO 2003

NEW MEXICO BUREAU OF GEOLOGY AND MINERAL RESOURCES
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Fifteen volcanic/hypabyssal rocks from the Alborz Mountains, Iran were submitted for
4 0 Ar/ 3 9 Ar dating by Drs. Bernard Guest and Gary Axen. It was hoped that the 4 0 Ar/ 3 9 Ar ages will help constrain the tectonic evolution of the Alborz Mountains. The table below lists the material/mineral prepared from each sample:
Groundmass concentrate
19-133-1
19-135-1
19-137-1
19-141-2B
20-49-2
3-89
Biotite
19-141-2A
19-141-2B
20-49-3
BG-4 84-2A
BG-4 84-2B
Hornblende
19-57-2
19-141-1
3-87
3-88
BG-4 86-3
40
39
The groundmass concentrate, biotite and hornblende samples were analyzed by the furnace incremental heating age spectrum 4 0 Ar/ 3 9 Ar method. Abbreviated analytical methods for the furnace sample is given in Table 1. Details of the overall operation of the New Mexico Geochronology
Research Laboratory are provided in the Appendix. Figures 1-16 show the age spectrum and inverse isochron yielded by the groundmass concentrates, biotites and hornblendes. A summary of the preferred ages yielded in this study is shown in Table 1.
Each of the six groundmass concentrate samples yielded a slightly to somewhat discordant age spectrum. For 19-133-1 (Figure 1), the first two heating steps (steps A and B) are significantly older
(age ≈ 60 Ma) than the remainder of the heating steps (age ≈ 32 Ma), which exhibit a concordant age distribution. The radiogenic yields increase gradually from a low of 2.3% for the lowest temperature heating step to greater than 84% for the 1250°C (the 1650°C or fusion step typically has a lower radiogenic yield resulting from a higher extraction line blank). The K/Ca values range from 0.03 (step
I) to 0.8 (step G), which are consistent with a basaltic groundmass concentrate. A weighted mean value for the flattest portion of the age spectrum (steps C through H; 85.7% of the cumulative 3 9 Ar
K released) yields an apparent age of 32.76±0.44 Ma (two sigma) with an unacceptable MSWD of 3.3.
The inverse isochron for 19-133-1 does however yield an acceptable MSWD of 1.8. The age yielded by the inverse isochron is 32.45±0.32 Ma with a 4 0 Ar/ 3 6 Ar intercept of 300±2.
Groundmass concentrate 19-135-1 (Figure 2) yields an age spectrum that is also discordant for the first few heating steps. While steps A and B are anomalously older than the remainder of the age spectrum, steps C and D are anomalously younger than the remainder of the age spectrum. This undulatory behavior is confined to the heating steps below 800°C, as those steps above 800°C are

isochronous about 32 Ma. The radiogenic yields and K/Ca values for 19-135-1 are nearly identical to those observed for 19-133-1. The weighted mean for the flattest portion of the age spectrum (steps E through I; 90.2% of the cumulative 3 9 Ar
K
released) yields an apparent age of 32.74±0.58 Ma with an unacceptable MSWD of 5.6. The inverse isochron yields an age of 32.5±2.2 Ma (MSWD=7.5) with a 4 0 Ar/ 3 6 Ar intercept of 298±11.
Groundmass concentrate 19-137-1 (Figure 3) yields the most well-behaved age spectrum of all the groundmass concentrate samples. Only the first three steps show any anomalous behavior and thus are not included in the plateau weighted mean age (8.72±0.11 Ma; 87% of the 3 9 Ar
K
released).
The radiogenic yields are slightly lower (<41%) than the previous samples but do increase from low to high temperature. The K/Ca ratios range from 0.06 to 2.3. The inverse isochron yields an age of
8.66±0.22 Ma with a 4 0 Ar/ 3 6 Ar intercept of 296±3 (MSWD=2.5).
The age spectrum for groundmass concentrate 19-141-2B (Figure 4) is very discordant, exhibiting what is commonly called a “saddle-shaped” spectrum. Apparent ages for the lowest temperature steps (A through C) and the highest temperature steps (G through I) are older (ranging from 8.5 to 27 Ma) than the steps in the middle of the age spectrum (D through F; ~7.3 Ma). The radiogenic yields range from 6% to 52% while the K/Ca ratios range from 0.5 to 5.8. A weighted mean for the three youngest heating steps (D through F) yields an age of 7.26±0.10 Ma
(MSWD=0.1). The inverse isochron is highly discordant when all of the heating steps are plotted
(MSWD>1000), but drops to an acceptable value when only four of the steps are plotted (A, D, E and
F; MSWD=0.2). The intercept age for the acceptable isochron is 7.04±0.12 Ma and the 4 0 Ar/ 3 6 Ar intercept is 304±2.
The age spectrum for groundmass concentrate 20-49-2 (Figure 5) exhibits anomalous behavior at the end of its heating schedule, rather than at the beginning. The first six heating steps (A through
F) are nearly isochronous at approximately 0.33 Ma. The remaining three heating steps range in age from 0.07 to 1.57 Ma. The radiogenic yields for this sample are low (<18%), while K/Ca ratios are higher (~3) than those observed in previous samples. The weighted mean age for the first six steps
(A-F) is 0.33±0.05 Ma (72.9% of the 3 9 Ar
K
released) with an unacceptable MSWD of 3.1. The inverse isochron age is 0.33±0.14 Ma with a 4 0 Ar/ 3 6 Ar ratio of 295±9 (MSWD=5.5).
The age spectrum for the 3-89 groundmass concentrate sample (Figure 6) only exhibits very small scale discordance at the beginning and end of the heating schedule. Radiogenic yields and K/Ca values are very similar to those observed for the previous groundmass concentrate (20-49-2). A plateau weighted mean age for the flattest portion of the age spectrum yields an age of 0.25±0.02 Ma
(79.2% of the 3 9 Ar
K
) with an acceptable MSWD of 0.6. The inverse isochron age is 0.25±0.02 with a
4 0 Ar/ 3 6 Ar ratio of 296±2 and a MSWD of 1.1.
The five biotite age spectra yield more precise results than the six groundmass concentrate samples. Biotite 19-141-2A yields a very flat age spectrum (Figure 7) with an apparent plateau over
2

90% of 3 9 Ar
K
released. Radiogenic yields range from about 40% to greater than 90% for the majority of the spectrum. K/Ca values are initially high (>10), but then drop below 10 for those temperature steps 1180°C and higher. The weighted mean age for the flattest portion of the age spectrum (steps C through K) is 6.87±0.08 Ma (95.6% of the 3 9 Ar
K
released) with a MSWD of 1.5. The inverse isochron yields a very similar age of 6.89±0.03 Ma ( 4 0 Ar/ 3 6 Ar intercept=294±2; MSWD=1.3).
Biotite 19-141-2B yields an age spectrum (Figure 8) very similar to that of 19-141-2A.
However, while the radiogenic yields are only slightly higher for 19-141-2B, the K/Ca values are approximately 4 times higher than those observed for 19-141-2A. The weighted mean age for steps D through K is 6.89±0.05 with 90.9% of the 3 9 Ar
K
released (MSWD=1.4). The inverse isochron yields an age of 6.91±0.06 Ma ( 4 0 Ar/ 3 6 Ar intercept=294±2; MSWD=1.5).
The age spectrum for biotite 20-49-3 (Figure 9) does exhibit some discordance, but still yields a precise weighted mean age. The age spectrum discordance is mainly confined to the lower temperature heating steps (<1180°C) where ages range from 0.23 to 0.92 Ma (excluding the A step).
Following the anomalous behavior at low temperatures, the age spectrum becomes more isochronous where ages range from 0.41 to 0.59 Ma. The radiogenic yields for 20-49-3 are low and consistent for a significant quantity of the gas released, ranging from 1.3 to about 10%. Only steps J and K have radiogenic yields greater than 20%. The K/Ca values are greater for the lower temperature steps (~12) than the high temperature steps (<6). A weighted mean for the flattest portion of the age spectrum
(steps G through K) yields an age of 0.51±0.03 Ma with 86.8% of the 3 9 Ar
K
released (MSWD=2.7).
The inverse isochron yields an age of 0.49±0.08 Ma ( 4 0 Ar/ 3 6 Ar intercept=296±6; MSWD=2.2).
The final two biotite samples from this study (BG-4 84-2A and BG-4 84-2B; Figures 10 and
11, respectively) yield results very similar to one another. In both cases, initial ages are anomalously younger than subsequent ages. For BG-4 84-2A, the youngest age (excluding the A step) is 4.69 Ma, while BG-4 84-2B’s youngest age is 4.82±0.86 Ma. Radiogenic yields and K/Ca values for the two samples are also very similar, positively correlating to the shape of the age spectra. The weighted mean age for BG-4 84-2A is 7.06±0.08 Ma (steps E through K) with 89.0% of the 3 9 Ar
K
released
(MSWD=2.9). The weighted mean age for BG-4 84-2B is 7.31±0.07 Ma (steps D through K) with
92.0% of the 3 9 Ar
K
released (MSWD=1.5). The inverse isochron for BG-4 84-2A yields an age of
7.07±0.13 Ma ( 4 0 Ar/ 3 6 Ar intercept=295±10; MSWD=2.6). The inverse isochron for BG-4 84-2B yields an age of 7.33±.010 Ma ( 4 0 Ar/ 3 6 Ar intercept=293±6; MSWD=2).
The hornblende samples from this study yield poor 4 0 Ar/ 3 9 Ar results when compared to the groundmass concentrates and the biotites. In nearly every case, the hornblendes degassed the bulk of their argon in only 2 to 3 heating steps. For hornblende 19-57-2 (Figure 12), approximately 85% of the 3 9 Ar
K
was released in steps H and I. The initial heating steps (A through G) yielded ages ranging from 2.50 to 21.36 Ma. Radiogenic yields range from 9.6 to 33.5%. K/Ca ratios range from 0.006 to
3

2.4. The weighted mean age for steps H and I is 1.39±0.21 Ma with a MSWD of 9.9. The inverse isochron yields an age of 0.69±2.3 Ma with a 4 0 Ar/ 3 6 Ar intercept of 360±246 (MSWD=42).
Hornblende 19-141-1 (Figure 13) also degassed predominantly in two heating steps, but its age spectrum does not exhibit the discordance present in 19-57-2. Steps H and I contain almost 95% of the 3 9 Ar
K
released for a weighted mean age of 6.71±0.08 Ma (MSWD=0.2). The radiogenic yields for those steps containing greater than 2% of the 3 9 Ar
K
are greater than 33%. The K/Ca ratios for all of the steps is consistent at approximately 0.15. The inverse isochron yields an age of 6.74±0.08 Ma
( 4 0 Ar/ 3 6 Ar intercept=289±3; MSWD=1.2).
Like the 19-141-2B groundmass concentrate, the 3-87 hornblende age spectrum (Figure 14) is distinctly saddle-shaped. Although the initial ages (steps A through C) appear to be consistent at approximately 9.5 Ma, the following 3 steps (D through F) immediately spike to ages in excess of 11
Ma. Steps G through I return to a comparatively isochronous distribution with a weighted mean age of 2.85±0.83 (77.4% of the 3 9 Ar
K
released) and a high MSWD of 41.9. The final two heating step once again increase in age to greater than 30 Ma. Radiogenic yields and K/Ca values are positively correlated to the shape of the age spectrum. The results for the inverse isochron are very discordant when all of the heating steps are plotted (MSDW=940). With only the three plateau steps plotted, the isochron yields an age of 3.5±1.8 ( 4 0 Ar/ 3 6 Ar intercept=274±43; MSWD=9.4).
The age spectrum results for 3-88 (Figure 15) exhibit some anomalous behavior at the intermediate heating steps. Steps D through G are slightly older and less precise than the rest of the age spectrum. However, this gas only represents ~10% of the total 3 9 Ar
K
released and therefore does not significantly influence the results of a weighted mean from steps B through I (age=0.34±0.32;
94.8% of the 3 9 Ar
K
; MSWD=4.7). Radiogenic yields for sample 3-88 are very low and in some cases are negative (over corrected for extraction line blank). K/Ca ratios range from 1.0 to less than 0.1.
The inverse isochron also yields very imprecise results (age=0.32±0.35 Ma; 4 0 Ar/ 3 6 Ar intercept=296±16; MSWD=3.8).
The age spectrum for the BG-4 86-3 hornblende (Figure 17) is very similar to that of the 19-
141-1 hornblende. Steps G through I yield a weighted mean age of 6.67±0.08 Ma with 95.8% of the
3 9 Ar
K
released and a MSWD of 0.2. Radiogenic yields for steps G through I range from 52 to 86% while the K/Ca ratios are relatively constant at approximately 0.1. The inverse isochron yields an age of 6.67±0.08 Ma with a 4 0 Ar/ 3 6 Ar intercept of 295±4 and a MSWD of 0.7.
For the vast majority of samples dated in this report, the weighted mean or plateau age is interpreted to represent the age of eruption of the rock/mineral in question. The one exception to this is groundmass concentrate sample 19-133-1, where the inverse isochron yields the preferred age
4

(32.45±0.32 Ma). The first two heating steps for 19-133-1 are significantly older than the remainder of the age spectrum. These anomalously old ages are likely caused by either small amounts of xenocrystic contamination or excess argon. Excess argon is non-atmospheric 4 0 Ar within a sample that is derived by a process other than the in situ radioactive decay of 4 0 K (McDougall and Harrison,
1999). Most commonly, excess argon refers to trapped 4 0 Ar/ 3 6 Ar compositions greater than 295.5 (the present day 4 0 Ar/ 3 6 Ar composition). In the case of the 19-133-1 groundmass concentrate, small amounts of excess argon may have been incorporated into glass and/or mineral phases at elevated argon partial pressures (i.e. at depth or in a magma chamber). In many cases, an inverse isochron is employed to test for trapped 4 0 Ar/ 3 6 Ar compositions greater than 295.5. The inverse isochron for the
19-133-1 groundmass concentrate yields a trapped 4 0 Ar/ 3 6 Ar composition (300±2) only slightly greater than 295.5. The slightly elevated 4 0 Ar/ 3 6 Ar trapped component prevents an unequivocal assessment of the scale of excess argon contamination, if it is present at all. However, given that the isochron data yields in an acceptable MSWD (1.8) while the plateau does not (3.3), we conclude that the inverse isochron age (32.45±0.32 Ma) is the best estimate for the age of eruption for this sample.
The age spectra for groundmass concentrate sample 19-135-1 and, to a lesser extent, 19-137-1 suggest minor amounts of 4 0 Ar loss. The anomalously young ages for the initial heating steps, coupled with the very low radiogenic yields for those steps, indicate the presence of alteration/hydration products. Despite the existence of this alteration, the majority of the age spectrum for both 19-135-1 and 19-137-1 does not appear to be greatly influenced. Therefore, we interpret the weighted mean ages for 19-135-1 (32.74±0.58 Ma) and 19-137-1 (8.72±0.11 Ma) as the preferred age of these samples.
The 19-141-2B exhibits the characteristic age spectrum shape often attributed to excess argon.
Assuming that the oldest ages in 19-141-2B are those most influenced by excess argon and that the youngest ages are those least influenced by excess argon, it is desirable to assign an age to a sample based solely on the youngest age. In the case of 19-141-2B, the youngest age is yielded by the weighted mean of steps D through F (7.26±0.10 Ma). However, it is difficult to ascertain the quantity of excess argon that may or may not be present in steps D, E and F, especially given the poor quality of the inverse isochron results. Therefore, while we state that the weighted mean age for this sample is the preferred age, we must also state that this age should be considered a maximum age. The presumed contamination by excess argon is supported by a biotite mineral separate from the 19-141-
2B sample (see below). The 19-141-2B groundmass concentrate yields a plateau weighted mean age approximately 350,000 years older than the 19-141-2B biotite plateau weighted mean age. Given that there are no signs of argon loss (e.g. alteration) in the biotite age spectrum, the source of the age discrepancy is undoubtedly the presence of excess argon in the 19-141-2B groundmass concentrate.
Excess argon also appears to be influencing the higher temperature heating steps of the 20-49-
2 groundmass concentrate age spectrum. However, unlike the 19-141-2B age spectrum, the lower
5

temperature steps of the 20-49-2 age spectrum do not appear to be significantly influenced by excess argon. Therefore, the preferred age of 20-49-2 is the plateau weighted mean age of 0.33±0.05 Ma.
Groundmass concentrate sample 3-89 does not appear to be influenced by either excess argon or alteration products (as indicated by both the shape of the age spectrum and the close agreement between the plateau weighted mean age and the inverse isochron age). Therefore, the preferred age of this sample is the plateau weighted mean age of 0.25±0.02 Ma.
For each of the five biotite samples in this study, the plateau weighted mean age yields the best estimate for the age of eruption for each respective sample.
Biotite Sample Age ± Error (2 s )
19-141-2A
19-141-2B
20-49-3
BG-4 84-2A
6.87±0.08 Ma
6.89±0.05 Ma
0.51±0.03 Ma
7.06±0.08 Ma
BG-4 84-2B 7.31±0.07 Ma
Only two biotite age spectra suggest any anomalous behavior: BG-4 84-2A and BG-4 84-2B. Both of these age spectra yield anomalously young ages in the lowest temperature heating steps. Like groundmass concentrate samples 19-135-1 and 19-137-1, biotites BG-4 84-2A and BG-4 84-2B may have small amounts of alteration products (chlorite?) present. However, despite the presumed presence of some alteration products, any 4 0 Ar loss seems to be confined to the lowest temperature portions of the samples. Therefore, the plateau weighed mean ages are still the preferred ages for these samples.
Although the hornblende samples were degassed in only 2 or 3 heating steps (as opposed to the 5 to 9 steps for most of the groundmass concentrate and biotite samples from this study) and despite some minor discordance with their age spectra, we interpret the hornblende plateau weighted mean ages as being the best estimates of their age of eruption.
Hornblende Sample
19-57-2
19-141-1
3-87
3-88
BG-4 86-3
Age ± Error (2 s )
1.39±0.21 Ma
6.71±0.08 Ma
2.85±0.83 Ma
0.34±0.32 Ma
6.67±0.08 Ma
Many of the disturbances observed in the hornblende age spectra can be attributed to excess argon.
For example, in sample 19-57-2, the age spike at the beginning of the age spectrum is probably the result of mineral inclusions or groundmass exposed on the surface of the hornblende separate, as suggested by the increase in K/Ca ratio for those steps. The same can also be said for the low temperature steps for hornblende sample 3-87. The highest temperature steps (J and K) for 3-87 may also be caused by mineral inclusions. In the case of hornblende 3-87, where a plateau is straddled by higher ages (similar to groundmass 19-141-2B) it is difficult to assess how much influence the excess
6

argon is imparting on the plateau age. Therefore, like 19-141-2B, we must conclude that the age of 3-
87 is a maximum age.
Figure 17 shows an age summary plot for the sixteen samples dated in this study. The most striking feature of the data is the apparent grouping of samples into three distinct pulses of magmatism. The earliest phase determined from this sample suite is at approximately 32.5 Ma, as recorded by samples 19-133-1 and 19-135-1. Following the 32.5 Ma activity, about 25 million years passes before more magmatic activity is recorded at approximately 8.7 to 6.7 Ma, as recorded by seven samples (eight with the 19-141-2B duplicate). At least another 3 million years of no activity passes until the final pulse begins at approximately 2.8 Ma, as recorded by six samples. It must be noted, however, that sampling procedures as well as incomplete exposures can potentially result in an apparent cluster of magmatic activity.
7

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Taylor, J.R., 1982. An Introduction to Error Analysis: The Study of Uncertainties in Physical
Measurements,. Univ. Sci. Books, Mill Valley, Calif., 270 p.
McDougall, I., and T. M. Harrison, 1988, Geochronology and thermochronology by the 4 0 Ar/ 3 9 Ar method: Oxford Monographs on Geology and Geophysics, v. 9, p. 212.
York, D., 1969. Least squares fitting of a straight line with correlated errors, Earth and Planet. Sci.
Lett., 5, 320-324.
8

Table 1. 40 Ar/ 39 Ar summary table and analytical procedures.
Sample
19-133-1
19-135-1
19-137-1
19-141-2B
20-49-2
3-89
19-141-2A
19-141-2B
20-49-3
BG-4 84-2A
BG-4 84-2B
19-57-2
19-141-1
3-87
3-88
BG-4 86-3
L# Irrad
** MSWD outside 95% confidence interval
Mineral
52210 NM-137 48.78 mg groundmass concentrate
52211 NM-137 46.98 mg groundmass concentrate
52208 NM-137 47.52 mg groundmass concentrate
52209 NM-137 49.83 mg groundmass concentrate
52291 NM-137 42.43 mg groundmass concentrate
52224 NM-137 33.31 mg groundmass concentrate
52192 NM-137 8.54 mg biotite
52194 NM-137 10.46 mg biotite
52196 NM-137 8.96 mg biotite
52245 NM-137 9.88 mg biotite
52199 NM-137 4.01 mg biotite
52197 NM-137 22.18 mg hornblende
52289 NM-137 21.70 mg hornblende
52195 NM-137 14.71 mg hornblende
52198 NM-137 12.78 mg hornblende
52193 NM-137 21.26 mg hornblende age analysis n % isochron 8
39 Ar n.a.
MSWD
2
K/Ca n.a.
Age ±2 s
32.45
0.32
plateau 5 plateau 5 plateau 3
90.2
87.0
23.2
5.6**
1.5
0.1
0.4
0.9
2.0
32.74
8.72
7.26
0.58
0.11
0.10
plateau 5 plateau 4 plateau 9 plateau 8 plateau 5 plateau 7
72.9
79.2
95.6
90.9
86.8
89.0
3.1**
0.6
1.5
1.4
2.7
2.9**
3.0
2.5
14.4
58.7
4.5
9.2
0.33
0.25
6.87
6.89
0.51
7.06
0.05
0.02
0.08
0.05
0.03
0.08
plateau 8 plateau 2 plateau 2 plateau 3 plateau 8 plateau 3
92.0
85.1
94.2
77.4
94.8
95.8
1.5
9.9**
0.2
41.9**
4.7**
0.2
5.8
0.1
0.1
0.1
0.3
0.1
7.31
1.39
6.71
2.85
0.34
6.67
0.07
0.21
0.08
0.83
0.32
0.08
Notes:
Sample preparation and irradiation:
Samples provided by Drs. Bernard Guest and Gary Axen of the University of Los Angeles, California.
Groundmass concentrate was separated using standard techniques (crushing, sieving, magnetics and hand-picking).
Biotite and hornblende were separated using standard techniques (crushing, sieving, magnetics, heavy liquids and hand-picking).
Samples were packaged and irradiated in machined Al discs for 7 hours in D-3 position, Texas A&M University Research Reactor.
Neutron flux monitor Fish Canyon Tuff sanidine (FC-1). Assigned age = 27.84 Ma (Deino and Potts, 1990)
relative to Mmhb-1 at 520.4 Ma (Samson and Alexander, 1987).
Instrumentation:
Mass Analyzer Products 215-50 mass spectrometer on line with automated all-metal extraction system.
All samples were step-heated in a double-vacuum resistance furnace.
Reactive gases from the biotites, horblendes and groundmass concentrates were removed during 12 to15 minute reaction with 3 SAES GP-50 getters,
2 operated at ~450°C and 1 at 20°C. Gas also exposed to a W filament operated at
~2000°C and a cold finger operated at -140°C.
Analytical parameters:
Electron multiplier sensitivity averaged 1.3x10
-16 moles/pA for those samples analyzed by the furnace.
Total system blank and background for the incrementally heated samples averaged 2440, 5.9, 7.3, 4.8, 15.6 x 10 -18 moles
J-factors determined to a precision of ± 0.1% by CO
2
laser-fusion of 4 single crystals from each of 4 or 6 radial positions around the irradiation tray.
Correction factors for interfering nuclear reactions were determined using K-glass and CaF
2
and are as follows:
( 40 Ar/ 39 Ar)
K
= 0.0002±0.0003; ( 36 Ar/ 37 Ar)
Ca
= 0.00028±0.000011; and ( 39 Ar/ 37 Ar)
Ca
= 0.00089±0.00003.
Age calculations:
Total gas ages and errors calculated by weighting individual steps by the fraction of 39 Ar released.
MSWD values are calculated for n-1 degrees of freedom for plateau and preferred ages.
Isochron ages, 40 Ar/ 36 Ar i
and MSWD values calculated from regression results obtained by the methods of York (1969).
Decay constants and isotopic abundances following Steiger and Jäger (1977).
All final errors reported at ±2s, unless otherwise noted.

Table 2. 40Ar/39Ar step-heating results for the groundmass concentrate, biotite and hornblende samples.
ID Temp
(°C)
40 Ar/ 39 Ar 37 Ar/ 39 Ar 36 Ar/ 39 Ar 39 Ar
K
(x 10
-3
) (x 10
-16
mol)
K/Ca
40 Ar*
(%)
39 Ar
(%)
19-133-1, 48.78 mg groundmass concentrate, J=0.0006998±0.10%, NM-137, Lab#=52210-01
D
E
F
A
B
C
625
700
750
800
875
975
2719.1
1114.7
207.8
135.7
88.09
54.40
G
H
1075
1250
38.53
30.77
I 1650 total gas age plateau
41.32
MSWD=3.3** isochron MSWD=1.8
1.902
1.648
1.510
1.412
1.301
0.9766
0.6375
4.375
20.80
n=9 n=6 n=8
8994.8
3604.9
612.1
368.6
206.8
94.51
8.76
21.5
15.0
22.0
33.2
35.3
42.83
17.44
51.3
98.1
63.20
12.2
297.4
steps C-H 254.8
40 Ar/ 36 Ar=300±2*
0.27
0.31
0.34
0.36
0.39
0.52
0.80
0.12
0.025
0.36
0.38
2.3
4.5
13.0
19.8
30.7
48.8
2.9
10.2
15.2
22.6
33.8
45.6
75.8
61.6
33.9
33.67
33.90
33.24
67.3
84.4
62.9
95.9
32.45
32.60
59.0
100.0
30.95
36.2
85.7
32.76
32.45
19-135-1, 46.98 mg groundmass concentrate, J=0.0006995±0.10%, NM-137, Lab#=52211-01
A
B
C
D
E
625
700
750
800
875
16837.9
465.2
215.1
163.0
133.1
F
G
H
975
1075
1250
56.40
52.80
I 1650 total gas age plateau
33.47
49.48
MSWD=5.6** isochron MSWD=7.5**
2.355
1.581
1.181
1.127
1.347
0.9240
0.5856
1.882
20.70
n=9 n=5 n=9
55747.2
1468.6
649.0
468.0
359.2
101.9
89.23
1.29
7.88
8.13
12.5
26.3
24.2
44.0
161.3
25.25
89.31
19.1
304.8
steps E-I 275.0
40
Ar/
36
Ar=298±11*
0.22
0.32
0.43
0.45
0.38
0.55
0.87
0.27
0.025
0.39
0.39
2.2
6.7
10.9
15.2
20.3
0.4
410.6
3.0
5.7
39.1
29.4
9.8
18.4
31.04
33.80
46.8
50.2
26.4
40.8
33.00
33.13
32.76
78.2
50.1
93.7
100.0
31.48
34.44
90.2
32.74
32.5
19-137-1, 47.52 mg groundmass concentrate, J=0.0006997±0.10%, NM-137, Lab#=52208-01
G
H
E
F
C
D
A
B
625
700
750
800
875
975
1075
1250
736.2
139.9
95.05
69.10
42.97
26.59
19.40
17.05
I 1650 total gas age plateau
54.36
MSWD=1.5
isochron MSWD=2.5
0.4789
0.2822
0.8515
1.355
0.5659
0.2253
0.2377
2.768
9.202
n=9 n=5 n=9
2448.5
452.1
300.5
209.6
121.5
66.34
42.73
35.01
12.6
25.8
28.5
40.9
54.7
62.0
86.4
277.6
160.8
10.8
599.2
steps D-H 521.5
40 Ar/ 36 Ar=296±3*
1.1
1.8
0.60
0.38
0.90
2.3
2.1
0.18
0.055
0.87
0.85
1.7
4.5
6.7
10.5
16.5
26.4
35.0
40.7
2.1
6.4
11.2
18.0
27.1
37.4
51.9
98.2
14.0
100.0
87.0
Age
(Ma)
16.0
7.98
7.99
9.16
8.95
8.83
8.556
8.744
9.63
8.87
8.72
8.66
±1 s
(Ma)
3.5
0.66
0.48
0.34
0.23
0.16
0.098
0.077
0.39
0.48*
0.11*
0.22*
69.4
2.3
1.4
0.94
0.60
0.31
0.26
0.16
0.31
1.28*
0.58*
2.2*
15.0
5.8
1.0
0.78
0.44
0.26
0.19
0.15
0.32
2.4*
0.44*
0.32*

Table 2. 40Ar/39Ar step-heating results for the groundmass concentrate, biotite and hornblende samples.
ID Temp
(°C)
40 Ar/ 39 Ar 37 Ar/ 39 Ar 36 Ar/ 39 Ar 39 Ar
K
(x 10
-3
) (x 10
-16
mol)
K/Ca
40 Ar*
(%)
39 Ar
(%)
Age
(Ma)
±1 s
(Ma)
19-141-2B, 49.83 mg groundmass concentrate, J=0.0006999±0.10%, NM-137, Lab#=52209-01
F
G
C
D
A
B
E
625
700
750 plateau
800
875
975
1075
I
H 1250
1650 total gas age isochron
182.5
22.25
14.52
12.05
11.08
11.77
17.47
18.14
47.58
MSWD=0.10
MSWD=0.2
0.0923
0.0883
0.1364
0.1964
0.2935
0.2613
0.1656
0.2743
1.115
n=9 n=3 n=4
581.3
45.58
24.88
21.46
18.04
20.39
36.42
34.12
88.48
38.9
131.4
53.0
35.4
52.6
79.7
108.1
200.9
24.4
724.4
steps D-F 167.8
40 Ar/ 36 Ar=304±2*
5.5
5.8
3.7
2.6
1.7
2.0
3.1
1.9
0.46
3.1
2.0
5.9
39.5
49.4
47.5
52.1
49.0
38.5
5.4
23.5
30.8
35.7
43.0
54.0
68.9
44.5
96.6
45.2
100.0
13.54
11.06
9.03
7.21
7.275
7.269
8.470
10.171
27.00
10.07
23.2
7.26
7.04
20-49.2, 42.43 mg groundmass concentrate, J=0.0007659±0.09%, NM-137, Lab#=52291-01
D
E
F
A
B
C
625
700
750
800
875
975 isochron
361.7
3.040
2.232
1.637
1.486
2.791
G
H
1075
1250
I 1650 total gas age plateau
10.49
15.11
20.36
MSWD=3.1**
MSWD=5.5**
0.3606
0.2294
0.2044
0.1757
0.1578
0.1645
0.2438
1.707
2.730
n=9 n=5 n=8
1220.4
9.754
6.867
4.920
4.178
8.628
1.34
52.0
47.1
86.0
168.6
136.1
35.41
47.77
67.90
73.9
67.7
38.8
671.6
steps B-F 489.8
40 Ar/ 36 Ar=295±9*
1.4
2.2
2.5
2.9
3.2
3.1
2.1
0.30
0.19
2.4
3.0
0.3
5.8
9.9
12.1
17.8
9.1
0.2
7.9
15.0
27.8
52.9
73.1
0.5
7.5
84.1
94.2
2.5
100.0
72.9
3-89, 33.31 mg groundmass concentrate, J=0.0007034±0.10%, NM-137, Lab#=52224-01
F
G
I
H
C
D
E
A
B
625
700
750
800 plateau
875
975
1075
1250
1650 total gas age isochron
3541.6
5.791
2.173
1.409
1.066
1.213
5.403
13.34
14.30
MSWD=0.6
MSWD=1.1
0.2500
11988.1
0.2217
0.1773
0.1594
0.1477
0.1635
0.2808
1.030
3.688
n=9 n=4 n=8
18.87
6.655
4.402
3.012
3.473
17.83
44.73
47.99
0.223
18.7
19.8
44.0
90.6
149.7
90.6
63.1
20.8
497.5
steps E-H 394.0
40 Ar/ 36 Ar=296±2*
2.0
2.3
2.9
3.2
3.5
3.1
1.8
0.50
0.14
2.5
2.5
0.0
4.0
10.2
8.6
17.6
0.0
3.8
7.8
16.6
34.8
16.5
2.9
64.9
83.1
1.5
95.8
2.9
100.0
79.2
1.5
0.244
0.304
0.273
0.365
0.352
3.2
0.051
0.037
0.027
0.017
0.028
0.065
1.57
0.071
0.12
0.72
0.14
0.45
0.11*
0.33
0.05*
0.33
0.14*
-1.1
0.297
0.280
0.153
0.238
26.0
0.079
0.063
0.030
0.016
0.254
0.199
0.012
0.044
0.26
0.54
0.10
0.14
0.25
0.11*
0.25
0.02*
0.25
0.02*
0.89
0.11
0.11
0.12
0.087
0.075
0.085
0.086
0.28
0.28*
0.10*
0.12*

Table 2. 40Ar/39Ar step-heating results for the groundmass concentrate, biotite and hornblende samples.
ID Temp
(°C)
40 Ar/ 39 Ar 37 Ar/ 39 Ar 36 Ar/ 39 Ar 39 Ar
K
(x 10
-3
) (x 10
-16
mol)
K/Ca
19-141-2A, 8.54 mg biotite, J=0.0006943±0.10%, NM-137, Lab#=52192-01
D
E
F
A
B
C
650
750
850
920
1000
1075
586.2
130.6
26.21
12.52
12.68
13.02
I
G
H
1110
1180
1210
13.54
10.93
7.512
J
K
1250
1300
6.511
6.023
L 1650 total gas age plateau
15.70
MSWD=1.5
isochron MSWD=1.3
0.1809
0.3387
0.0469
0.0259
0.0313
0.0463
0.0468
0.0743
0.0892
0.0796
0.0165
0.1334
n=12 n=9 n=12
1969.5
424.1
70.75
24.22
24.08
25.86
27.77
18.84
6.779
3.332
1.725
33.00
2.22
5.60
14.9
19.4
25.0
28.5
16.3
22.7
19.7
43.3
44.2
2.84
244.6
steps C-K 234.0
40 Ar/ 36 Ar=294±2*
10.9
6.9
5.7
6.4
31.0
3.8
13.8
14.4
2.8
1.5
10.9
19.7
16.3
11.0
19-141-2B, 10.46 mg biotite, J=0.0006941±0.10%, NM-137, Lab#=52194-02
E
F
G
H
A
B
C
D
650
750
850
920
1000
1075
1110
1180
2170.7
272.5
46.53
11.42
10.47
10.00
8.944
8.019
I
J
1210
1250
K
L
1300
1650 total gas age plateau
7.093
6.276
5.921
11.21
MSWD=1.4
isochron MSWD=1.5
0.2716
0.0368
0.0187
0.0102
0.0120
0.0116
0.0280
0.0204
0.0150
0.0079
0.0049
0.1901
n=12 n=8 n=12
7396.8
909.7
137.9
19.92
16.54
15.79
11.45
8.460
0.343
8.02
15.8
30.7
30.2
29.2
20.3
29.0
5.391
2.454
1.446
18.21
21.3
44.1
71.7
3.44
304.0
steps D-K 276.4
40 Ar/ 36 Ar=294±2*
1.9
13.9
27.3
50.2
42.6
43.9
18.2
25.0
33.9
64.5
104.0
2.7
55.2
58.7
20-49-3, 8.96 mg biotite, J=0.0006929±0.10%, NM-137, Lab#=52196-01
I
J
K
G
H
E
F
C
D
A
B
650
750
850
920
1000
1075
1110
1180
1210
1250
1300
64.24
19.79
13.56
9.726
5.584
4.948
4.634
5.717
3.152
1.922
1.601
L 1650 total gas age plateau
14.31
MSWD=2.7
isochron MSWD=2.2
0.1274
0.1602
0.1178
0.0697
0.0412
0.0424
0.0416
0.0826
0.0984
0.1904
0.2212
0.1934
n=12 n=5 n=12
213.4
64.89
45.32
30.45
17.84
15.95
14.49
17.78
9.590
5.103
4.168
46.25
steps G-K
2.13
3.02
3.55
3.74
7.14
11.5
10.7
44.7
54.0
80.3
24.8
1.50
246.9
214.3
40 Ar/ 36 Ar=296±6*
4.0
3.2
4.3
7.3
12.4
12.0
12.3
6.2
5.2
2.7
2.3
2.6
5.1
4.5
40 Ar*
(%)
0.7
4.1
20.2
42.9
43.9
41.3
39.4
49.1
73.4
45.8
55.1
63.1
85.0
91.6
80.8
98.8
38.0
100.0
0.9
3.2
9.3
17.2
27.5
39.1
95.6
1.9
3.2
1.3
7.5
5.7
4.8
7.7
8.2
10.4
22.4
24.2
39 Ar
(%)
0.9
2.1
3.5
5.0
7.9
12.6
16.9
35.0
56.9
89.4
99.4
4.6
100.0
86.8
Age
(Ma)
5.2
6.70
6.63
6.71
6.96
6.73
6.67
6.71
6.896
6.917
6.894
7.45
6.81
6.87
6.89
±1 s
(Ma)
-0.7
1.3
12.4
48.5
53.3
53.4
62.2
68.8
77.6
88.5
60.8
75.3
92.8
98.9
52.2
100.0
0.1
-18.9
2.8
7.9
18.0
4.6
7.21
6.918
28.0
37.6
44.2
53.8
6.979
6.673
6.953
6.899
90.9
6.875
6.938
6.866
7.31
6.82
6.89
6.91
15.6
1.5
0.35
0.094
0.083
0.087
0.079
0.064
0.068
0.047
0.040
0.28
0.28*
0.05*
0.06*
1.49
0.78
0.23
0.92
0.40
0.297
0.443
0.587
0.408
0.537
0.484
0.49
3.8
0.93
0.25
0.11
0.11
0.10
0.13
0.11
0.062
0.047
0.041
0.32
0.28*
0.08*
0.03*
0.88
0.36
0.33
0.27
0.17
0.087
0.099
0.062
0.038
0.022
0.040
0.82
0.58
0.51
0.14*
0.51
0.03*
0.08*

Table 2. 40Ar/39Ar step-heating results for the groundmass concentrate, biotite and hornblende samples.
ID Temp
(°C)
40 Ar/ 39 Ar 37 Ar/ 39 Ar 36 Ar/ 39 Ar 39 Ar
K
(x 10
-3
) (x 10
-16
mol)
K/Ca
40 Ar*
(%)
39 Ar
(%)
Age
(Ma)
±1 s
(Ma)
BG-4 84-2A, 9.88 mg biotite, J=0.0007058±0.10%, NM-137, Lab#=52245-01
I
G
H
C
D
A
B
E
F
650
750
850
920
1000
1075
1110
1180
1210
885.4
92.21
33.67
12.16
8.146
7.631
8.773
8.499
7.412
J
K
1250
1300
6.878
6.349
L 1650 total gas age plateau
10.23
MSWD=2.9** isochron MSWD=2.6**
0.9152
1.790
0.3421
0.0586
0.0284
0.0452
0.0668
0.1256
0.1180
0.2486
0.0530
0.1822
n=12 n=7 n=9
2994.7
300.1
101.5
23.50
8.492
6.648
10.73
10.06
6.493
1.49
3.13
5.75
14.9
41.7
61.3
25.0
25.9
21.6
4.645
2.936
32.1
47.5
16.06
6.34
286.6
steps E-K 255.0
40 Ar/ 36 Ar=295±10*
0.56
0.29
1.5
8.7
17.9
11.3
7.6
4.1
4.3
2.1
9.6
2.8
8.8
9.2
BG-4 84-2B, 4.01 mg biotite, J=0.0006913±0.10%, NM-137, Lab#=52199-01
E
F
G
H
A
B
C
D
650
750
850
920
1000
1075
1110
1180 isochron
1447.0
40.33
41.32
14.18
MSWD=2
8.052
7.765
7.889
8.871
I
J
1210
1250
K
L
1300
1650 total gas age plateau
7.763
7.433
6.500
10.91
MSWD=1.5
0.7216
0.7916
0.4477
0.0987
0.0441
0.0616
0.0995
0.1471
0.1794
0.3566
0.1060
0.1413
n=12 n=8 n=12
4880.0
123.6
123.5
28.66
6.979
6.496
7.160
9.931
0.317
2.09
2.00
4.64
13.2
20.5
9.64
11.0
6.037
5.490
2.502
17.74
9.30
12.5
10.7
3.55
99.4
steps D-K 91.4
40 Ar/ 36 Ar=293±6*
0.71
0.64
1.1
5.2
11.6
8.3
5.1
3.5
2.8
1.4
4.8
3.6
5.5
5.8
0.1
4.0
11.0
42.9
69.2
74.3
63.9
65.1
74.2
0.3
9.6
11.8
40.3
74.4
75.3
73.3
67.1
77.2
78.6
88.8
52.1
0.5
1.6
3.6
8.8
23.4
44.7
53.5
62.5
70.0
80.3
86.4
81.2
97.8
53.7
100.0
89.0
0.3
2.4
4.4
9.1
22.4
43.0
52.7
63.8
73.1
85.7
96.4
100.0
92.0
0.6
4.69
4.73
6.63
7.166
7.205
7.127
7.034
6.994
7.024
6.972
6.98
6.96
7.06
7.07
19-57-2, 22.18 mg hornblende, J=0.0006923±0.10%, NM-137, Lab#=52197-01
F
G
H
C
D
A
B
E
800
850
950
1020
1080
1120
1160
1200
54.10
4.704
13.65
53.29
85.94
40.02
21.00
7.172
I
J
1300
1400
K 1650 total gas age plateau
3.394
27.40
47.82
MSWD=9.9** isochron MSWD=42**
0.3507
0.2168
0.4419
1.206
2.072
2.977
4.125
4.350
5.357
87.34
10.82
n=11 n=2 n=11
125.0
9.212
33.48
159.9
258.7
95.96
62.77
22.58
9.140
99.01
149.3
2.76
1.97
1.71
0.727
0.485
0.587
5.21
21.7
60.8
0.640
0.344
97.0
steps H-I 82.5
40 Ar/ 36 Ar=360±246*
1.5
2.4
1.2
0.42
0.25
0.17
0.12
0.12
0.095
0.006
0.047
0.21
0.10
31.8
42.5
27.8
11.5
11.2
29.8
13.3
12.0
2.9
4.9
6.7
7.4
7.9
8.5
13.9
36.2
21.36
2.50
4.73
7.7
12.0
14.8
3.49
1.08
33.5
19.6
99.0
99.6
9.6
100.0
1.424
7.1
5.8
2.34
85.1
1.39
0.69
6.3
4.82
6.07
7.12
7.461
7.283
7.197
7.41
7.46
7.271
7.182
7.08
7.22
7.31
7.33
13.1
0.86
0.67
0.28
0.085
0.073
0.095
0.11
0.10
0.079
0.091
0.27
0.34*
0.07*
0.10*
6.3
0.96
0.33
0.14
0.059
0.054
0.077
0.072
0.056
0.055
0.044
0.14
0.22*
0.08*
0.13*
0.72
0.33
0.51
1.2
1.8
1.4
0.35
0.10
0.035
1.3
2.2
0.28*
0.21*
2.3*

Table 2. 40Ar/39Ar step-heating results for the groundmass concentrate, biotite and hornblende samples.
ID Temp
(°C)
40 Ar/ 39 Ar 37 Ar/ 39 Ar 36 Ar/ 39 Ar 39 Ar
K
(x 10
-3
) (x 10
-16
mol)
K/Ca
40 Ar*
(%)
39 Ar
(%)
Age
(Ma)
±1 s
(Ma)
19-141-1, 21.70 mg hornblende, J=0.0007079±0.10%, NM-137, Lab#=52289-01
F
G
D
E
I
H
A
B
C
800
850
950
1020
1080
1120
1160
1200
1300
2353.0
987.5
104.1
36.46
40.30
33.30
13.82
8.884
6.270
J
K
1400
1650 total gas age plateau
15.08
25.92
MSWD=0.2
isochron MSWD=1.2
5.998
26.06
5.111
3.169
2.441
2.708
3.308
3.488
3.704
4.232
7.183
n=11 n=2 n=11
8086.5
3431.9
326.4
123.4
125.3
96.76
31.97
13.13
4.490
34.78
73.95
0.207
0.169
0.207
0.262
0.341
0.402
2.79
9.29
107.2
1.99
0.863
123.7
steps H-I 116.5
40
Ar/
36
Ar=289±3*
0.085
0.020
0.100
0.16
0.21
0.19
0.15
0.15
0.14
0.12
0.071
0.14
0.14
-1.5
-2.5
7.8
0.7
8.6
14.8
33.6
59.6
83.7
34.2
99.3
18.0
100.0
0.2
-46.9
0.3
-32.1
0.5
10.4
0.7
1.0
1.3
3.5
11.1
97.7
0.3
4.5
6.3
5.93
6.76
6.709
94.2
6.58
6.0
6.53
6.71
6.74
22.1
10.3
4.1
2.9
2.4
1.9
0.36
0.13
0.041
0.44
1.0
0.28*
0.08*
0.08*
3-87, 14.71 mg hornblende, J=0.0006937±0.10%, NM-137, Lab#=52195-01
F
G
D
E
H
A
B
C
800
850
950
1020
1080
1120
1160
1200
65.19
24.27
16.92
29.85
32.35
37.78
16.71
21.69
I
J
1300
1400
K 1650 total gas age plateau
6.408
31.90
31.08
MSWD=41.9** isochron MSWD=9.4**
0.7779
0.4178
0.5865
0.7778
0.9718
2.372
3.790
4.200
4.819
4.490
1.046
n=11 n=3 n=3
194.9
55.66
31.62
47.13
46.71
96.62
50.52
70.99
14.60
26.68
20.88
2.69
1.94
2.47
1.39
1.22
1.79
9.90
18.9
27.0
2.55
2.28
72.1
steps G-I 55.8
40 Ar/ 36 Ar=274±43*
0.66
1.2
0.87
0.66
0.53
0.22
0.13
0.12
0.11
0.11
0.49
0.22
0.12
D
E
F
3-88, 12.78 mg hornblende, J=0.0006918±0.10%, NM-137, Lab#=52198-01
A
B
C
800
850
950
65.54
7.641
4.825
1.188
0.4953
0.5846
220.9
26.77
17.08
2.63
3.30
4.08
0.43
1.0
0.87
1020
1080
1120
8.042
17.09
24.57
0.8558
1.537
2.282
24.45
57.21
87.57
2.49
0.861
0.534
0.60
0.33
0.22
G
H
1160
1200
32.50
25.11
I
J
1300
1400
K 1650 total gas age plateau
7.756
82.65
138.1
MSWD=4.7** isochron MSWD=3.8**
3.476
4.062
5.218
24.99
14.43
n=11 n=8 n=11
105.9
86.27
2.32
8.50
26.51
284.8
493.3
31.5
0.226
0.108
56.5
steps B-I 53.6
40 Ar/ 36 Ar=296±16*
0.15
0.13
0.098
0.020
0.035
0.26
0.25
11.8
32.4
45.0
53.6
57.6
25.0
12.5
4.9
38.9
76.5
80.4
100.0
0.6
-3.0
-3.6
11.0
1.8
-4.5
3.7
6.4
9.8
11.8
13.5
16.0
29.7
55.9
93.3
96.8
77.4
4.7
10.5
17.7
22.1
23.6
24.6
4.6
-0.2
28.7
43.7
4.6
0.7
99.4
99.8
-4.7
100.0
94.8
9.58
9.81
9.51
19.91
23.18
11.78
2.62
1.33
3.128
30.36
31.04
5.95
2.85
3.5
0.46
-0.29
-0.22
1.11
0.39
-1.4
1.86
-0.06
0.444
0.7
-8.2
0.33
0.34
0.32
0.78
0.26
0.20
0.31
0.86
1.2
0.58
0.28
0.077
3.2
7.3
0.48*
0.32*
0.35*
0.83
0.60
0.39
0.67
0.74
0.61
0.19
0.18
0.074
0.40
0.41
0.46*
0.83*
1.8*

Table 2. 40Ar/39Ar step-heating results for the groundmass concentrate, biotite and hornblende samples.
ID Temp
(°C)
40 Ar/ 39 Ar 37 Ar/ 39 Ar 36 Ar/ 39 Ar 39 Ar
K
(x 10
-3
) (x 10
-16
mol)
K/Ca
40 Ar*
(%)
39 Ar
(%)
Age
(Ma)
±1 s
(Ma)
BG-4 86-3, 21.26 mg hornblende, J=0.0006943±0.10%, NM-137, Lab#=52193-01
F
G
D
E
H
A
B
C
800
850
950
1020
1080
1120
1160
1200
428.6
533.0
241.2
104.7
51.85
24.32
10.05
9.175
I
J
1300
1400
K 1650 total gas age plateau
6.208
14.45
36.58
MSWD=0.2
isochron MSWD=0.7
1.672
3.086
4.167
4.429
4.582
4.755
4.773
4.683
4.736
5.067
4.337
n=11 n=3 n=11
1434.1
1838.6
762.2
343.8
151.3
64.40
17.54
14.46
4.306
34.07
107.2
0.478
0.051
0.072
0.080
0.233
0.559
3.14
15.5
63.8
1.63
0.549
86.1
steps G-I 82.4
40 Ar/ 36 Ar=295±4*
0.31
0.17
0.12
0.12
0.11
0.11
0.11
0.11
0.11
0.10
0.12
0.11
0.11
1.2
-1.9
6.8
3.3
14.5
23.4
52.3
57.7
85.8
33.2
97.5
99.4
14.4
100.0
0.6
6.3
0.6
-12.7
0.7
20.4
0.8
1.1
1.7
5.4
23.4
4.4
9.4
7.1
6.60
6.636
95.8
6.682
6.02
6.6
6.66
6.67
6.67
4.2
14.8
10.3
8.2
3.0
1.2
0.21
0.099
0.043
0.52
1.3
0.28*
0.08*
0.08*
Notes:
Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for interferring reactions.
Individual analyses show analytical error only; plateau and total gas age errors include error in J and irradiation parameters.
Analyses in italics are excluded from final age calculations.
Discrimination = 1.00684±0.00098 a.m.u.
†=analyses excluded from plateau weighted mean age.
ø=analyses excluded from inverse isochron age.
K/Ca = molar ratio calculated from reactor produced 39 Ar
K
and 37 Ar
Ca
.
* 2 s error
** MSWD outside of 95% confidence interval

L# 52210: 19-133-1, 48.78 mg groundmass concentrate
80
40
0
35
30
25
0
60
55
50
45
A
B
700
40
32.76 ± 0.44 Ma
C
750
10
D
800
20
E
875
F
975
G
1075
Integrated Age = 36.2 ± 2.4 Ma
H
1250
30 40 50 60 70
Cumulative 39 Ar
K
Released
80
I
90 100
1
0.1
0.01
0.0034
0.0032
0.003
0.0028
0.0026
0.0024
A
B
C
D
E
0.0022
0.002
0.0018
0.0016
0.0014
0.0012
0.001
0.0008
0.0006
0.0004
0.0002
0
0
Isochron age = 32.45 ± 0.32 Ma
40 Ar/ 36 Ar Intercept = 300 ± 2
MSWD = 1.8, n = 8
0.005
0.01
0.015
F
I
G
0.02
39 Ar/ 40 Ar
0.025
0.03
H
0.035
0.04
Figure 1.
40 Ar/ 39 Ar age spectrum and inverse isochron for the 19-133-1 groundmass concentrate.
The preferred age of this sample is the inverse isochron age (32.45±0.32 Ma). All errors are 2
σ
.

L# 52211: 19-135-1, 46.98 mg groundmass concentrate
80
40
0
44
42
40
38
36
34
32
30
28
26
B
24
22
20
0
C
750
D
800
10
E
875
20
F
975
G
1075
32.74 ± 0.58 Ma
H
1250
Integrated Age = 34.44 ± 1.28 Ma
30 40 50 60
Cumulative 39 Ar
K
70
Released
80
0.0034
0.0032
0.003
A
0.0028
0.0026
0.0024
0.0022
0.002
0.0018
0.0016
0.0014
0.0012
0.001
0.0008
0.0006
0.0004
0.0002
0
0
B
C
D
E
Isochron age = 32.5 ± 2.2 Ma
40 Ar/ 36 Ar Intercept = 298 ± 11
MSWD = 7.5, n = 9
0.005
0.01
0.015
F
G
I
0.02
39 Ar/ 40 Ar
0.025
H
0.03
90
I
100
0.035
0.04
1
0.1
0.01
Figure 2.
40
Ar/
39
Ar age spectrum and inverse isochron for the 19-135-1 groundmass concentrate.
The preferred age of this sample is the plateau weighted mean age (32.74±0.58 Ma). All errors are 2
σ
.

L# 52208: 19-137-1, 47.52 mg groundmass concentrate
80
40
0
24
22
20
18
16
14
12
10
8
6
4
2
0
0
B C
750
D
800
10 20
E
875
F
975
G
8.72 ± 0.11 Ma
1075
H
1250
30
Integrated Age = 8.87 ± 0.48 Ma
40 50 60 70 80
Cumulative 39 Ar
K
Released
I
90 100
10
1
0.1
0.01
0.0034
0.0032
0.003
0.0028
0.0026
0.0024
A
B
C D
I E
F
0.0022
0.002
0.0018
0.0016
0.0014
0.0012
G
H
0.001
0.0008
0.0006
0.0004
0.0002
0
0
Isochron age = 8.66 ± 0.22 Ma
40 Ar/ 36 Ar Intercept = 296 ± 3
MSWD = 2.5, n = 9
0.02
0.04
0.06
0.08
39 Ar/ 40 Ar
0.1
0.12
0.14
Figure 3.
40 Ar/ 39 Ar age spectrum and inverse isochron for the 19-137-1 groundmass concentrate.
The preferred age of this sample is the plateau weighted mean age (8.72±0.11 Ma). All errors are 2
σ
.

L# 52209: 19-141-2B, 49.83 mg groundmass concentrate
80
40
0
30
25 I
20
7.26 ± 0.10 Ma
15
10
A
625
5
0
0 10
B
700
20
C
750 D
800
E
875
F
975
G
1075
Integrated Age = 10.07 ± 0.28 Ma
30 40 50
Cumulative 39 Ar
K
60 70
Released
80
H
1250
90 100
0.0034
0.0032
0.003
0.0028
0.0026
0.0024
A
0.0022
0.002
0.0018
0.0016
0.0014
0.0012
I
B
H
G
C F
D
E
0.001
0.0008
0.0006
0.0004
0.0002
0
0
Isochron age = 7.04 ± 0.12 Ma
40 Ar/ 36 Ar Intercept = 304 ± 2
MSWD = 0.2, n = 4
0.02
0.04
0.06
0.08
0.1
39 Ar/ 40 Ar
0.12
0.14
0.16
0.18
10
1
0.1
Figure 4.
40 Ar/ 39 Ar age spectrum and inverse isochron for the 19-141-2B groundmass concentrate.
The preferred age of this sample is the plateau weighted mean age (7.26±0.10 Ma). All errors are 2
σ
.

L# 52291: 20-49.2, 42.43 mg groundmass concentrate
80
40
0
0.8
0.6
0.4
0.2
0
-0.2
-0.4
2
1.8
1.6
1.4
1.2
1
0
B
700
C
750
10
0.33 ± 0.05 Ma*
D
800
20 30
E
875
F
975
Integrated Age = 0.45 ± 0.11 Ma
G
40 50 60 70
Cumulative 39 Ar
K
Released
80
H
1250
90
I
100
10
1
0.1
0.0034
0.0032
0.003
A
G
I
H
F
B
C D
0.0028
0.0026
E
0.0024
0.0022
0.002
0.0018
0.0016
0.0014
0.0012
0.001
0.0008
0.0006
0.0004
0.0002
0
0
Isochron age = 0.33 ± 0.14 Ma
40 Ar/ 36 Ar Intercept = 295 ± 9
MSWD = 5.5, n = 8
0.5
1 1.5
2 2.5
39 Ar/ 40 Ar
3 3.5
4
Figure 5.
40 Ar/ 39 Ar age spectrum and inverse isochron for the 20-49-2 groundmass concentrate.
The preferred age of this sample is the plateau weighted mean age (0.33±0.05 Ma). All errors are 2
σ
.

L# 52224: 3-89, 33.31 mg groundmass concentrate
80
40
0
1.4
1.2
1
0.8
0.6
0.4
0.2
0
B
700
C
-0.2
-0.4
0 10
D
800
20
E
875
0.25 ± 0.02 Ma
F
975 G
1075
Integrated Age = 0.25 ± 0.11 Ma
30 40 50 60 70
Cumulative 39 Ar
K
Released
80
H
1250
90
I
100
10
1
0.1
0.0034
0.0032
A
H
G
I
B
0.003
0.0028
0.0026
0.0024
0.0022
0.002
C
F
D
E
0.0018
0.0016
0.0014
0.0012
0.001
0.0008
0.0006
0.0004
0.0002
0
0
Isochron age = 0.25 ± 0.02 Ma
40 Ar/ 36 Ar Intercept = 296 ± 2
MSWD = 1.1, n = 8
0.5
1 1.5
2 2.5
3
39 Ar/ 40 Ar
3.5
4 4.5
5
Figure 6.
40 Ar/ 39 Ar age spectrum and inverse isochron for the 3-89 groundmass concentrate.
The preferred age of this sample is the plateau weighted mean age (0.25±0.02 Ma). All errors are 2
σ
.

L# 52192: 19-141-2A, 8.54 mg biotite
80
40
0
11
10
9
8
6.87 ± 0.08 Ma
7
6
5
B
4
C
850
D
920
E
1000
3
0 10 20
F
1075
G
1110
H
1180
I
1210
J
1250
Integrated Age = 6.81 ± 0.28 Ma
30 40 50 60 70
Cumulative 39 Ar
K
Released
80
K
1300
L
90 100
100
10
1
0.0034
0.0032
0.003
0.0028
0.0026
0.0024
0.0022
0.002
0.0018
0.0016
0.0014
0.0012
0.001
0.0008
0.0006
0.0004
0.0002
0
0
A
B
Isochron age = 6.89 ± 0.03 Ma
40 Ar/ 36 Ar Intercept = 294 ± 2
MSWD = 1.3, n = 12
0.02
C
0.04
L
0.06
G
E
F
D
0.08
H
0.1
39 Ar/ 40 Ar
I
J
K
0.12
0.14
0.16
0.18
Figure 7.
40 Ar/ 39 Ar age spectrum and inverse isochron for the 19-141-2A biotite. The preferred age of this sample is the plateau weighted mean age (6.87±0.08 Ma). All errors are 2
σ
.

L# 52194: 19-141-2B, 10.46 mg biotite
80
40
0
10
9
8
6.89 ± 0.05 Ma
7
6
C
850
D
920
E
1000 F
1075
G
1110
H
1180
I
1210
J
1250
5
4
0 10 20 30
Integrated Age = 6.82 ± 0.28 Ma
40 50 60
Cumulative 39 Ar
K
70
Released
80
K
1300
90 100
1000
100
10
1
0.0034
0.0032
0.003
0.0028
0.0026
0.0024
0.0022
0.002
0.0018
0.0016
0.0014
0.0012
A
B
C
L
D
E
F
0.001
0.0008
0.0006
0.0004
0.0002
0
0
Isochron age = 6.91 ± 0.06 Ma
40 Ar/ 36 Ar Intercept = 294 ± 2
MSWD = 1.5, n = 12
0.02
0.04
0.06
0.08
0.1
39 Ar/ 40 Ar
G
H
I
J K
0.12
0.14
0.16
0.18
Figure 8.
40 Ar/ 39 Ar age spectrum and inverse isochron for the 19-141-2B biotite. The preferred age of this sample is the plateau weighted mean age (6.89±0.05 Ma). All errors are 2
σ
.

L# 52196: 20-49-3, 8.96 mg biotite
80
40
0
100
10
1
2
1.5
1
0.51 ± 0.03 Ma
0.5
0
E
1000
F
G
1110
H
1180
-0.5
-1
0 10 20
I
1210
J
1250
30
Integrated Age = 0.51 ± 0.14 Ma
40 50 60 70
Cumulative 39 Ar
K
Released
80 90
K
1300
100
0.0034
0.0032
0.003
0.0028
0.0026
0.0024
0.0022
0.002
0.0018
0.0016
0.0014
0.0012
D E
F
H G
I
J
K
0.001
0.0008
0.0006
0.0004
0.0002
Isochron age = 0.49 ± 0.08 Ma
40
Ar/
36
Ar Intercept = 296 ± 6
MSWD = 2.2, n = 12
0
0 0.2
0.4
0.6
0.8
1 1.2
1.4
1.6
1.8
2.
2.2
2.4
2.6
39 Ar/ 40 Ar
Figure 9.
40 Ar/ 39 Ar age spectrum and inverse isochron for the 20-49-3 biotite. The preferred age of this sample is the plateau weighted mean age (0.51±0.03 Ma). All errors are 2
σ
.

L# 52245: BG-4-84-2A, 9.88 mg biotite
80
40
0
10
9
8
7
6
5
4
C
3
0
D
920
E
1000
10 20
F
1075
7.06 ± 0.08 Ma
G
1110
H
1180
I
1210
J
1250
K
1300
L
30
Integrated Age = 6.96 ± 0.22 Ma
40 50 60 70
Cumulative 39 Ar
K
Released
80 90 100
100
10
1
0.1
0.0034
0.0032
0.003
0.0028
0.0026
0.0024
0.0022
0.002
0.0018
0.0016
0.0014
0.0012
0.001
0.0008
0.0006
0.0004
0.0002
0
0
A
B
Isochron age = 7.07 ± 0.13 Ma
40
Ar/ 36 Ar Intercept = 295 ± 10
MSWD = 2.6, n = 9
0.02
C
0.04
0.06
D
0.08
L
0.1
G
H
E
F
I
J
K
0.12
0.14
0.16
0.18
39 Ar/ 40 Ar
Figure 10.
40
Ar/
39
Ar age spectrum and inverse isochron for the BG-4 84-2A biotite. The preferred age of this sample is the plateau weighted mean age (7.06±0.08 Ma). All errors are 2
σ
.

L# 52199: BG-4 -84-2B, 4.01 mg biotite
80
40
0
G
1110
H
1180
I
1210
J
1250
K
1300 L
100
10
1
0.1
10
9
8
7
6
5
4
C
D
920
E
1000
3
0 10 20
F
1075
7.31 ± 0.07 Ma*
30
Integrated Age = 7.22 ± 0.34 Ma
40 50 60 70
Cumulative 39 Ar
K
Released
80 90 100
0.002
0.0018
0.0016
0.0014
0.0012
0.001
0.0008
0.0006
0.0004
0.0002
0.0034
0.0032
0.003
0.0028
0.0026
0.0024
0.0022
0
0
A
0.02
B
C
Isochron age = 7.33 ± 0.10 Ma
40 Ar/ 36 Ar Intercept = 293 ± 6
MSWD = 2, n = 12
0.04
0.06
D
0.08
L
0.1
39 Ar/ 40 Ar
H
E
G
F
I
J
0.12
K
0.14
0.16
Figure 11.
40 Ar/ 39 Ar age spectrum and inverse isochron for the BG-4 84-2B biotite. The preferred age of this sample is the plateau weighted mean age (7.31±0.07 Ma). All errors are 2
σ
.

L# 52197: 19-57-2, 22.18 mg hornblende
80
40
0
25
20
A
15
10
5
0
C
B
850
G
1160
-5
0 10
1.39 ± 0.21 Ma
20
H
1200
30
I
1300
Integrated Age = 2.34 ± 0.28 Ma
40 50 60 70
Cumulative 39 Ar
K
Released
80 90 100
10
0.1
0.001
0.0028
J
0.0026
F
0.0024
0.0022
A
C
0.002
0.0018
0.0016
0.0014
0.0012
B
I
0.001
0.0008
0.0006
0.0004
0.0002
0
0
Isochron age = 0.69 ± 2.3 Ma
40 Ar/ 36 Ar Intercept = 360 ± 246
MSWD = 42, n = 11
0.2
0.4
0.6
0.8
1
39 Ar/ 40 Ar
1.2
1.4
1.6
1.8
Figure 12.
40 Ar/ 39 Ar age spectrum and inverse isochron for the 19-57-2 hornblende.
The preferred age of this sample is the plateau weighted mean age (1.39±0.21 Ma). All errors are 2
σ
.

L# 52289: 19-141-1, 21.70 mg hornblende
80
40
0
12
11
10
9
8
7
6
5
4
3
2
0
G
H
1200
10
6.71 ± 0.08 Ma
I
1300
J
20
Integrated Age = 6.53 ± 0.28 Ma
30 40 50
Cumulative
60
39 Ar
K
70
Released
80 90
0.0036
B
0.0034
0.0032
A
0.003
0.0028
0.0026
0.0024
C
0.0022
0.002
0.0018
0.0016
0.0014
0.0012
0.001
0.0008
0.0006
0.0004
0.0002
0
0
E
F
D
K
J
G
Isochron age = 6.74 ± 0.08 Ma
40 Ar/ 36 Ar Intercept = 289 ± 3
MSWD = 1.2, n = 11
H
I
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
39 Ar/ 40 Ar
100
1
0.1
0.01
Figure 13.
40 Ar/ 39 Ar age spectrum and inverse isochron for the 19-141-1 hornblende.
The preferred age of this sample is the plateau weighted mean age (6.71±0.08 Ma). All errors are 2
σ
.

L# 52195: 3-87, 14.71 mg hornblende
80
40
0
35
30
25
20
15
10
5
A B
850
C
0
D
F
-5
-10
0 10
2.85 ± 0.83 Ma
G
1160
20
H
1200
Integrated Age = 5.95 ± 0.46 Ma
30 40 50
Cumulative 39 Ar
K
60 70
Released
I
1300
80
J K
90 100
10
1
0.1
0.0038
0.0036
0.0034
0.0032
0.003
0.0028
A
H
G
0.0026
0.0024
0.0022
0.002
0.0018
0.0016
0.0014
0.0012
0.001
0.0008
0.0006
F
D
E
J
B
C
0.0004
0.0002
0
0
K
Isochron age = 3.5 ± 1.8 Ma
40 Ar/ 36 Ar Intercept = 274 ± 43
MSWD = 9.4, n = 3
0.05
0.1
0.15
I
0.2
39 Ar/ 40 Ar
0.25
0.3
0.35
Figure 14.
40
Ar/
39
Ar age spectrum and inverse isochron for the 3-87 hornblende.
The preferred age of this sample is the plateau weighted mean age (2.85±0.83 Ma). All errors are 2
σ
.

L# 52198: 3-88, 12.78 mg hornblende
80
40
0
8
6
4
0.34 ± 0.32 Ma
2
0
-2
A
800
B
-4
C
950
D
E
G
-6
0 10
H
1200
I
1300
20
F
1120
Integrated Age = 0.33 ± 0.48 Ma
30 40 50 60 70
Cumulative 39 Ar
K
Released
80 90 100
10
1
0.1
0.01
0.0034
0.0032
0.003
0.0028
0.0026
0.0024
0.0022
0.002
0.0018
0.0016
0.0014
0.0012
0.001
0.0008
0.0006
0.0004
0.0002
0
0
Isochron age = 0.32 ± 0.35 Ma
40 Ar/ 36 Ar Intercept = 296 ± 16
MSWD = 3.8, n = 11
0.5
1 1.5
2
39 Ar/ 40 Ar
2.5
3 3.5
4
Figure 15.
40
Ar/
39
Ar age spectrum and inverse isochron for the 3-88 hornblende.
The preferred age of this sample is the plateau weighted mean age (0.34±0.32 Ma). All errors are 2
σ
.

L# 52193: BG-4-86-3, 21.26 mg hornblende
80
40
0
10
9
8
7
6 G
5
4
3
0 10
H
1200
20
6.67 ± 0.08 Ma
I
1300
Integrated Age = 6.66 ± 0.28 Ma
30 40 50
Cumulative 39 Ar
K
60 70
Released
80 90 100
1
0.1
0.01
0.0034
0.0032
0.003
0.0028
C
B
A
D
E
0.0026
0.0024
0.0022
0.002
0.0018
0.0016
0.0014
0.0012
0.001
0.0008
0.0006
0.0004
0.0002
K
F
J
Isochron age = 6.67 ± 0.08 Ma
40
Ar/
36
Ar Intercept = 295 ± 4
MSWD = 0.7, n = 11
0
0 0.02
0.04
0.06
0.08
G
0.1
H
I
0.12
0.14
0.16
0.18
39 Ar/ 40 Ar
Figure 16.
40 Ar/ 39 Ar age spectrum and inverse isochron for the BG-4 86-3 hornblende.
The preferred age of this sample is the plateau weighted mean age (6.67±0.08 Ma). All errors are 2
σ
.

19-133-1
19-135-1
19-137-1
19-141-2B
20-49-2
3-89
19-141-2A
19-141-2B
20-49-3
BG-4 84-2A
BG-4 84-2B
19-57-2
19-141-1
3-87
3-88
BG-4 86-3
3 2 .4 5 ±0 .3 2 Ma
3 2 .7 4 ±0 .5 8 Ma
0
8 .7 2 ±0 .1 1 Ma
7 .2 6 ±0 .1 0 Ma
0 .3 3 ±0 .0 5 Ma
0 .2 5 ±0 .0 2 Ma
6 .8 7 ±0 .0 8 Ma
6 .8 9 ±0 .0 5 Ma
0 .5 1 ±0 .0 3 Ma
7 .0 6 ±0 .0 8 Ma
7 .3 1 ±0 .0 7 Ma
1 .3 9 ±0 .2 1 Ma
6 .7 1 ±0 .0 8 Ma
2 .8 5 ±0 .8 3 Ma
0 .3 4 ±0 .3 2 Ma
6 .6 7 ±0 .0 8 Ma
10 20
30 40
40
39